This invention relates to a digital filter for an electronic musical instrument and, more particularly, to a digital filter employing a pole filter capable of controlling a pole of amplitude-frequency characteristic and a zero filter capable of effecting zero control in combination, and further to a digital filter employing filters of different constructions in switchable combinations.
A tone color circuit in an electronic musical instrument which requires subtle characteristics have heretofore been constituted mostly of an analog circuit. An analog tone color circuit (particularly an analog filter), however, tend to become a large circuit. This is particularly the case when a tone color requiring a fixed formant (e.g., a human voice, wind instruments such as oboe and bassoon, piano and string instruments) must be realized, for a large number of analog filter circuits must be provided in parallel resulting in a circuit design of a large scale system. Further, since a digital tone signal cannot be applied directly to the analog tone color circuit, this type of circuit requires a further complicated circuitry if a digital tone generation circuit is to be combined to the analog tone color circuit.
To cope with this problem, an attempt has recently been made to employ a digital filter for a tone color circuit of an electronic musical instrument. According to the digital filter, a digital tone signal generated by a digital tone generation circuit can be applied directly and in addition, a simultaneous processing of plural tone signals can be attained on a time shared basis with a relatively small hardware construction. Hence, the digital filter is much more economical than the analog tone color circuit. In the digital filter, it is generally difficult to set a coefficient so as to obtain a desired amplitude-frequency characteristic. Particularly in the electronic musical instrument, a high fidelity realization of a desired fixed formant requires skilled controlling of locations (bands) and levels of "peak" and "valley" in the filter amplitude-frequency characteristic and this is very difficult to realize by simply applying a digital filter to a tone color circuit of an electronic musical instrument. There are several known basic types of digital filter but they are only capable of performing a control which is mainly directed either to the pole or zero and none of them is capable of simultaneously controlling both pole and zero skilfully and readily. In a digital filter of a type capable of mainly controlling the pole, for example, a plurality of poles are formed in different frequency regions and a valley portion is obtained only as a result of overlapping skirt portions of peaks thus formed. In this case, it is very difficult to set the location and level of the valley portion to desired states.
It is, therefore, an object of the present invention to provide a digital filter suitable for use in an electronic musical instrument which is capable of controlling relatively easily both the peak and valley portions in the amplitude-frequency characteristic.
The "peak" characteristic herein means an amplitude-frequency characteristic forming a pass band in the vicinity of the frequency corresponding to a pole, the "valley" characteristic an amplitude-frequency characteristic forming a stop band in the vicinity of the frequency corresponding to zero.
As described above, there are several basic types in the digital filter and digital filters of various constructions can be designed on the basis of these basic types. For example, one digital filter of the same basic type as another can be made a digital filter of a different construction by providing it with a different number of operation stages. If the basic type is different, a digital filter is naturally of a different construction. In utilizing a digital filter as a tone color circuit of an electronic musical instrument, it is desirable to employ a digital filter of a construction most suitable for the purpose of the tone color control. Since, however, a digital filer of a fixed construction cannot be selectively changed, there is the inconvenience that once a filter of a certain construction has been provided in a tone color circuit, it cannot be readily changed. Further, digital filters of various constructions are required depending upon the type of an electronic musical instrument. If a large number of digital filters of different constructions are prepared for each of various types of electronic musical instruments, this will be extremely uneconomical in the aspect of the manufacturing cost.
It is, therefore, another object of the present invention to provide a digital filter suitable for use in an electronic musical instrument which is capable of selectively realizing digital filters of various constructions by a single set of digital filter device.
It is still another object of the invention to provide a digital filter for an electronic musical instrument which is capable of selectively realizing various filter constructions by combining plural sets of digital filter devices of a common hardware construction and selectively switching the filter constructions of the respective filter devices, and which is also capable of contributing to reduction of the manufacturing cost by the unification of the hardware construction of the respective filter devices.